I haven't played without modded story progression in a long time, but this would be my guess, since I'm not sure which coding is EA's and what is the NRaas mod's as far as this goes.
Generally the boss in a career is assigned to a sim at a higher level than your sim. If your sim becomes the highest level sim in town in that career, it might reassign the boss either to a random sim in town, or, if a household in town owns the property or is a partial owner, it will go to a sim that has an ownership stake.
I remember once when I played the Landgraabs, Geoffrey was in the medical career, but it eventually made his wife Nancy his boss. I was confused because she was in the business career, but then I remembered the household had bought an ownership stake in the hospital.